Creating high poly from low poly model
To make it LOOK smooth, select the object, hit 'W' and select 'Shade Smooth'

To ADD POLYGONS, you can optionally select all faces (in face select mode) and hit 'W' then 'Subdivide' This will quadruple your face count. Once that's done, you might want to hit 'W' then 'Smooth' (which actually moves vertices to achieve a smoother mesh).

Or, you can go add a Subdivision Surface Object Modifier, adjust the 'view' and 'render' values until you like the mesh. Apply the modifier to get back to a fully editable mesh again.